Clean up `test_coverage_table_spec` file

What does this MR do?

Related to #299906 (closed)

Refactor the test_coverage_table_spec.js file to use more mock data.

Also refactored how the mock apollo data was handled.

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• [-] Style guides
• [-] Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done